A thermal cracking process comprises the steps of
(a) supplying the feed to the inlet of a cracking furnace, allowing the feed to crack in the coils of the cracking furnace in the presence of steam at elevated temperature and removing from the cracking furnace a cracked stream which is enriched in light olefins; PA1 (b) quenching the cracked stream; PA1 (c) supplying the cooled cracked stream to a fractionation column; and PA1 (d) removing from the top of the fractionation column a gaseous stream, from the side of the fractionation column a side stream containing fuel oil components and from the bottom of the fractionation column a bottom stream. PA1 (a) supplying the feed to the inlet of a membrane unit provided with a membrane, and removing from the permeate side a permeate and from the retentate side a retentate; PA1 (b) supplying the permeate to the inlet of a cracking furnace, allowing the permeate to crack in the coils of the cracking furnace in the presence of steam at elevated temperature and removing from the cracking furnace a cracked stream which is enriched in light olefins; PA1 (c) quenching the cracked stream; PA1 (d) supplying the cooled cracked stream to a fractionation column; PA1 (e) supplying the retentate to the fractionation column; and PA1 (f) removing from the top of the fractionation column a gaseous stream, from the side of the fractionation column a side stream of fuel oil components and from the bottom of the fractionation column a bottom stream.
This process is also called steam cracking, naphtha cracking or ethylene manufacturing.
The fractionation column is also called `primary fractionator`.
The gaseous stream removed from the top of the fractionation column comprises light olefins, such as ethylene and propylene, and other components, such as hydrogen, methane, C.sub.4 products and pyrolysis gasoline (C.sub.5.sup.+). Downstream of the fractionation column, the gaseous overhead is further treated to recover ethylene.
From the side of the fractionation column one or more side stream(s) is (are) removed which contains fuel oil components.
From the bottom of the fractionation column is removed a liquid bottom stream which contains heavy cracked fuel oil. Part of the liquid bottom stream is cooled and mixed with the cracked stream upstream of the fractionation column to quench this stream. The remainder is removed as heavy fuel oil.
Upstream of the fractionation column the feed is cracked in the cracking furnace. The liquid hydrocarbon feed is preheated upstream of the cracking furnace or inside the upper part of the cracking furnace. In the cracking furnace the liquid hydrocarbon stream is first vaporized and subsequently cracked. Vaporization of the liquid hydrocarbon stream takes place in the presence of steam in a vaporization coil located in the upper part of the cracking furnace, where the liquid is vaporized by the heat from the hot flue gas. The upper part of the cracking furnace is called the convection section. After the stream is vaporized, it enters into the pyrolysis coil in the radiant section of the cracking furnace. In the pyrolysis coil hydrocarbons are cracked in the presence of steam to obtain the desired product. This is well known, and the conditions for vaporization and cracking are well known as well.
Feeds that are used are naphtha (a straight-run gasoline fraction) and gas oil (a distillate, intermediate in character between kerosene and light lubricating oils). Such feeds, however, tend to become more expensive, and this triggers the interest in using other hydrocarbon feeds for the cracking process. Examples of such feeds are certain condensates which comprise naphtha and gas oil components. Condensate is a mixture of hydrocarbons which are sometimes produced with natural gas.
These feeds, however, also contain contaminants. In relation to the present invention two contaminants are of particular interest. On the one hand hydrocarbons with a high boiling point and on the other hand salts present in water droplets which are dispersed in the stream of light hydrocarbons.
Hydrocarbons with a high boiling points are hydrocarbons which do not easily vaporize, even in the presence of steam. Examples of such hydrocarbons are polynuclear aromatics, polynuclear cycloparaffins, large paraffinic hydrocarbons (waxes), and olefinic components such as polynuclear cycloolefins and large olefinic hydrocarbons specially diolefins. These high boiling point hydrocarbons are soluble in the light hydrocarbons, and the solution usually has a darker colour for example an ASTM colour of 3 or more, determined in accordance with ASTM D1500. An example of a contaminated liquid stream containing light hydrocarbons is a black condensate, which is a mixture of hydrocarbons which are sometimes produced with natural gas having an ASTM colour of 3 or more. The contaminated liquid may also include waste streams for the refinery.
The salts in the hydrocarbon streams will come from formation water or from other treatments at a refinery, examples of contaminating salts are sodium chloride, magnesium chloride, calcium chloride and iron chloride. Other salts, such as sulphates may be present as well.
These components will remain liquid in the vaporization coil and will foul the inner surface of the vaporization coil. Fouling by deposited components will reduce the heat transfer and will consequently adversely affect the performance of a steam cracker. Moreover, fouling can even cause plugging of the vaporization coil.
It is an object of the present invention to provide a process of producing light olefins in particular from contaminated feeds, wherein fouling of the vaporization coil is reduced.